The invention relates to a method and apparatus for setting registration in a multicolor printing machine having a number of exposure devices for the digital production of color separations, the actions of setting up and combining color separations being controlled, in order to set registration, such that in-register prints are obtained, influencing variables which are caused by printed pages and have an influence on registration being taken into account in setting up and combining color separations before the onset of a change in said influencing variables.
Printing colored illustrations, in particular color images, is carried out by a number of color separations being printed over one another. These are generally the colors yellow, magenta and cyan as well as black. If required, special colors are added. By overprinting these colors, all color combinations can be achieved, the quality of the prints depending significantly on the in-register overprinting of the color separations. In the case of digital printing processes, for example electrostatic printing processes, the maintenance of the registration of the overprint is achieved by the image production devices being controlled such that the color separations meet one another in-register when they are transferred to a printing substrate.
A method and an apparatus for a multicolor printing machine are disclosed by U.S. Pat. No. 5,689,757. The teaching of this document is to take account of the influence of the roughness of the printing substrate on the setting of machine parameters, specifically before the printing substrate whose roughness has been registered passes through the printing machine.
However, this teaching assumes that a printed page has run out of the machine before processing is started of a printed page which has different influencing variables, such as a different toner profile or a different printing substrate. Otherwise, the risk is that there will be registration faults in the transition phase from one printed page to the other. This is based on the fact that printing substrates from a previous printed page are still in the machine while one or more image cylinders are having images of color separations of a new printed page set, or that the printing units are printing different printed pages, if, for example, only one copy of each printed page is to be printed. In the last case, each color separation will also be set up only once. In the aforementioned cases, the printing substrate and the toner profile of a previous printed page determine the transmission of force between the carrier for printing substrates, the transfer cylinder and the image cylinder.
In the case of printing machines without an image transfer cylinder, the transmission of force is determined in a corresponding way without the latter, but likewise by the previous printing substrate and previous toner profile. If this transmission of force is not taken into account for setting the image of the new printed page on the image cylinder, a registration fault will always occur when influencing variables that are caused by the printed pages change, such as the paper grade, in particular the paper thickness, or the toner application. From the aforementioned, it can be seen that an ongoing printing process with different printed pages is not possible with a method and apparatus of the aforementioned prior art.
The invention is therefore based on the object of developing a method and apparatus for a multicolor printing machine such that no interruption to the printing process between two different printed pages is necessary, but rather that printing can be continued directly without any loss of quality. With respect to the invention, the object is achieved in that in the event of a registration correction resulting from a change in influencing variables caused by a printed page, the retroactive influence of the influencing variables of the preceding printed page is taken into account.
The advantage of the invention resides in the fact that the print quality is ensured even in the event of seamless continuous printing in the case of a sequence of different printed pages with changes in the printing substrate, the toner profile or other influencing variables caused by the printed pages. The influencing variables which are caused by the printed pages and have an influence on registration are taken into account at such an early timexe2x80x94including their interaction in the change-over area from one job to anotherxe2x80x94that no registration fault occurs any more because of them. A printing machine according to the invention can thus continue to print in spite of such a change, since the influence of changes on registration can be compensated for, without interrupting the printing, between the setting up of two color separations even when the new color separation is already being set up and the preceding color separation from the previous printed page, which differs from the following one, has not yet been transferred, or not yet completely transferred, to the printing substrate. The printing machine is therefore suitable for prints in which each printed page differs from the other in terms of image content and/or printing substrates. There are therefore no setting interruptions or reject prints, and the prints can be set up extremely economically with high quality.
Since each printed-page change runs through the machine from one printing unit to the next, and even within a printing unit runs through the printing unit on the way from production as far as transfer to the printing substrate, at the same time different machine configurations must be taken into account by variable influencing variables for each printing unit. There are various influencing variables which are caused by the printed pages and have an influence on registration. One of these is the composition of the paper. Here, the paper thickness influences the curved contact with a cylinder and therefore the speed of the cylinder if the latter, for example, is driven via the carrier for printing substrates. Such a cylinder may be an image cylinder or an image transfer cylinder, if such cylinders are inserted between the image cylinders and the carrier for printing substrates. However, the paper composition also has an effect on the contact pressure and the coefficient of friction, which in turn have an effect on registration.
A further influencing variable is the toner profile. This can also deform the elastic cover of an image transfer cylinder elastically, which has an influence on the transmission ratio. Furthermore, the coefficient of friction is influenced as a result, and likewise has an effect on the transfer. The influence of the toner profile is primarily of importance when elements carrying the image and substrate form a drive train, as is the case in most digitally operating multicolor printing machines. This influence is particularly high in the event of force transmission between image cylinder and image transfer cylinder. The influence of the toner profile on the transmission ratio also exists in the event of the transmission of force between substrate carrier and image transfer cylinder or between substrate carrier and image cylinder, that is to say in the case of machines which do not have any image transfer cylinders. The toner profile changes the effective radius and the coefficient of friction at these force-transmission contact positions as well.
The influence of properties of the printing substrate used, or the influence of the toner application selected can be taken into account as influencing variables caused by the printed pages. In order to be able to compensate for these influences on registration exactly, it is proposed that the influencing variables be taken into account on the basis of stored values based on experience. Although these values based on experience must be determined, for example via the detection of registration marks, since there is a limited number of paper grades, the necessary values based on experience are available following detection. Thereafter, the measure according to the invention, taking the influencing variables into account before these have an influence on the print, is possible. The storage of toner profiles is also possible, since the values based on experience can relate to the percentage of area coverage of the toner profile. With respect to the apparatus, it is proposed that it be equipped with at least one memory which contains stored values based on experience, the at least one controller controlling the actions of setting up and combining color separations on the basis of these values based on experience.
There are various possibilities for taking the influencing variables into account. Thus, provision can be made for the influencing variables to be taken into account as a result of activation by a manual input. With regard to the apparatus, provision is then made for it to be equipped with an input device, via which the influencing variables to be taken into account can be activated manually. A further expedient configuration of the method provides for the influencing variables to be taken into account on the basis of a stored file of printed pages. For the apparatus, a printing-machine control system is then provided which contains stored files of printed pages, and the influencing variables are taken into account on the basis of these files.
Finally, provision can be made for the influencing variables to be taken into account by the automatic registration of the same. With regard to the apparatus, provision is then made for a device for the automatic registration of influencing variables. One example of such automatic registration is the registration of paper thicknesses and taking account of values that are based on experience and assigned to these paper thicknesses. It is likewise possible for toner profiles to be registered by a measurement, for example by a densitometer, or in any other way, and for values based on experience and assigned to these toner profiles to be taken into account. With respect to the apparatus, provision can also be made for the device for registering influencing variables to register toner profiles from the digital image files, and for the at least one memory to contain values that are based on experience and assigned to the toner profiles.
Values based on experience for various paper grades and for various toner profiles are expediently available. With respect to the apparatus, at least one memory is provided, which may be loaded with influencing variable values based on experience for various paper grades and/or with values based on experience for various toner profiles. The use of the measure according to the invention to correct a registration control system is particularly expedient. Provision can therefore be made for the method to be used to correct a registration control system which is based on the detection of registration marks which are printed by the individual printing units, the basic setting of the registration being by the registration marks, and the changes in substrate and toner being carried out by the correction before changes caused by a printed page can have any influence on the registration marks. With respect to the apparatus, provision is made for at least one controller to be provided that implements the abovementioned method.
A further possibility is that the method is used to correct a registration control system which is based on the detection of the positions of the elements carrying color separations and printing substrates. With respect to the apparatus, provision is made for at least one controller to be provided to control registration by detecting the positions of the elements that carry the color separations and printing substrates by position detecting elements, and take the influencing variables into account as correction factors. Position detecting elements of this type can be, for example, rotary encoders.